Advancements in communication technologies have permitted the introduction, and widespread usage of, wireless communication systems. Cellular communication systems, as well as other types of multi-user, wireless communication systems, are regularly utilized by large numbers of consumers to communicate both voice and non-voice information.
A communication system is formed, at a minimum, of a sending station and a receiving station interconnected by way of a communication channel. In a wireless communication system, the communication channel formed between the sending and receiving stations is formed of a radio channel defined upon a portion of the electromagnetic spectrum. Because a radio channel is utilized to form a communication link between the sending and receiving stations, a wired connection conventionally required in a wireline communication system is obviated. Use of a wireless communication system to communicate therethrough, thereby, is permitted at, and between, locations at which the formation of a wireline connection would be impractical. Also, as the need for the wireline connection between the sending and receiving stations is obviated, the infrastructure costs associated with installation of a communication system rather than a conventional wireline communication system are reduced.
A cellular communication system is exemplary of a wireless, multi-user radio communication system. Cellular communication systems have been installed throughout wide geographical areas and have achieved wide levels of usage. A cellular communication system generally includes a fixed network infrastructure installed throughout the geographical area which is to be encompassed by the communication system. A plurality of fixed-site base stations are installed at selected positions throughout the geographical area. The fixed-site base stations are coupled, by way of additional portions of the network infrastructure to a public network, such as a PSTN (Public-Switched, Telephonic Network). Portable transceivers, referred to as mobile stations, communicate with the base stations by way of radio links.
Because of the spaced-apart positioning of the base stations, only relatively low-power signals are required to be generated by the mobile stations and by the base stations to effectuate communications therebetween. A cellular communication system, as a result, typically efficiently utilizes the portion of the electromagnetic spectrum allocated thereto upon which radio channels are defined. That is to say, because only low-power signals are required to be generated, the same radio channels can be reused at different locations throughout the geographical area encompassed by the communication system.
In an ideal communication system, a communication signal, when received at a receiving station, is substantially identical to the corresponding communication signal when transmitted by a sending station. However, in a non-ideal communication system in which the communication signal must be transmitted upon a non-ideal communication channel, the signal, when received at the receiving station, is dissimilar to the corresponding communication signal when sent by the sending station. Distortion of the communication signal caused during propagation of the communication signal causes such dissimilarities to result. If the distortion is significant, the informational content is the signal cannot accurately be recovered at the receiving station.
Fading caused by multi-path transmission, for instance Raleigh fading, might alter the values of the information bearing bits of the communication signal during its transmission upon the communication channel. Quasistatic flat fading, for example, models the situation when the fading is flat in frequency and is constant during the duration of a relevant block of transmitted symbols, usually referred to as a frame. In contrast, fast flat fading models the situation when the fading is flat in frequency but changes as fast as from a transmitted symbol epoch to the next. If the propagation distortion is not properly corrected, the communication quality levels of the communications are, at a minimum, reduced.
Various techniques are utilized to overcome distortion introduced upon a communication signal as a result of transmission upon a non-ideal communication channel.
The redundancy of the transmitted signal through time encoding of the signal, prior to its transmission, is sometimes utilized to counteract the distortion introduced upon the signal during its transmission upon the communication channel. By increasing the time redundancy of the signal, the likelihood that the informational content of the signal can be recovered, once received at the receiving station, is increased. Introducing time redundancy into the signal is sometimes referred to as creating time diversity.
Utilization of space diversity is also sometimes utilized to overcome distortion introduced upon the communication signal. Typically, space diversity refers to the utilization of more than one transmit antenna transducer from which a communication signal is transmitted, thereby to provide spatial redundancy. The antenna transducers must be separated by a distance great enough to ensure that the signals communicated from the respective antenna transducers fade in an uncorrelated manner.
Space and time diversity are sometimes utilized together, thereby further to enhance transmission diversity to combat signal fading caused, e.g., by multi-path transmission.
Combinations of both space and time coding further enhance transmission diversity to combat signal fading caused by multi-path transmission. At any symbol epoch, exactly one symbol is transmitted from each transmit antenna. Each transmitted symbol is selected from the constellation of signal points that characterizes the modulator associated with a particular antenna. Note that the constellations pertaining to the different transmit antennas can be in general different, but in practice it may be preferable to have identical signal constellations for all transmit antennas. The particular constellation points selected to be sent over the different transmit antennas during an arbitrary (multiple) transmission are appropriately determined from the encoder's output symbols. Trellis encoding is sometimes used to effectuate space time coding. But, block coding is valid too. In the former case, the selection of the constellation points, starting from the encoder's output symbols, is decided by a construction, referred to as a trellis, which describes all possible transitions between a given, finite number of states. The states are tuples of certain most recent symbols, e.g., bits, applied to the input of the trellis encoder. For example, if the input sequence consists of raw information bits, then the tuples reflect the most recent past history of the information bit sequence which is provided to the trellis encoder, and the trellis describes a transformation of an input sequence of bits, into an output sequence of symbols, referred to as a coded symbol sequence. Note that the coded symbols can be nonbinary, too. The trellis is represented by successive columns, comprised of all the valid states, and evolutions in time between states (in successive columns) are referred to as transitions. Each branch corresponds to a particular combination of new input symbols while in a given state. A mapper is utilized to map each coded symbol to a signal constellation point, thus determining the modulation parameters for a carrier signal.
In construction of the trellis and the mapper, a significant goal is to optimize the manner by which labels to trellis branches are assigned and to optimize the manner by which constellation points are assigned to the symbols used in the trellis branch labels. The optimality of the assignation is characterized in terms of a measure, referred to as a distance between two different codewords. The distance, ultimately, is determinative of the physically-meaningful, probability of a receiving station mistaking one codeword for another. The smaller the probability of a mistake, the better shall be the performance of a space-time code that is utilized in the effectuation of the communication. In order to ensure as large of a distance as possible between two codewords, a succession of points selected, during transmission, from the signal constellation, as dictated by the trellis, must be carefully determined during initial construction of the trellis. One approach to doing this is to maximize the minimum among all possible distances between pairs of transmitted codewords. To do this, codes are selected whose trellises have as large as possible pair wise distances between codewords. But, the distance spectrum is important too; it may be acceptable to accept a small minimum distance, if that distance occurs very seldomly.
A set of all signals that possibly can be selected for transmission upon a multiple number of transmit antennas, within a meaningful time interval and according to all possible patterns of input symbols, forms a space-time code. Subsequent to constructing the space-time code, the space-time code is implemented as an encoder at a sending station and as a decoder at a receiving station. A significant problem is to determine a manner by which to efficiently select points from a given signal constellation, in such a manner as to optimize an overall performance of the transmission scheme. Performance is defined, for instance, in terms of a Frame Error Probability (FEP).
The utilization of diversity to counteract the effects of fading, however, generally increases the bandwidth requirements of the radio channel to communicate the informational content of the communication signal to the receiving station. As bandwidth constraints upon the communication channel upon a radio communication system, as well as other types of communication systems limits the communication capacity of the system, efforts are generally also made to limit the bandwidth requirement to communicate information between a sending and a receiving station.
An increase in the diversity of the communication signal which requires, conventionally, an increase in bandwidth consumption to communicate a communication signal is contradictory to the competing goal of minimizing the amount of bandwidth required to communicate information between a sending and a receiving station.
If a manner could be provided by which to impart improved space-time redundancy to a communication signal without requiring an increase in the amount of bandwidth required to communicate a certain amount of information between a sending and receiving station, improved communication quality, for a given communication capacity, would result.
It is in light of this background information related to the communication of information between a sending and a receiving station that the significant improvements of the present invention have evolved.